Lithium silicate glass ceramic and glass with transition metal oxide

ABSTRACT

The invention relates to a lithium silicate glass ceramic, which contains at least 8.5 wt.-% transition metal oxide selected from the group consisting of oxides of yttrium, oxides of transition metals with an atomic number from 41 to 79 and mixtures of these oxides. The invention also relates to a corresponding lithium silicate glass, a process for the preparation of the glass ceramic and of the glass as well as their use.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of European Patent ApplicationSerial No. 10160222.5, filed Apr. 16, 2010 and European PatentApplication Serial No. 10168792.9, filed Jul. 7, 2010, which are herebyincorporated by reference in their entirety.

TECHNICAL FIELD

The invention relates to lithium silicate glass ceramics and glasseswith a high content of an element with a high atomic number, which aresuitable in particular for use as dental materials, for example for thepreparation of dental restorations.

BACKGROUND OF THE INVENTION

Lithium silicate glass ceramics are characterized by very goodmechanical properties, which is why they have been used for a long timein the dental field and primarily for preparing dental crowns and smallbridges. The known lithium silicate glass ceramics usually contain asmain components SiO₂, Li₂O, Al₂O₃, alkali metals such as Na₂O or K₂O andnucleating agents such as P₂O₅. In addition, they can contain as furthercomponents for example further alkali metal oxides and/or alkaline earthmetal oxides and/or ZnO. Glass ceramics are also known which containsmall quantities of further metal oxides and in particular colouring andfluorescent metal oxides.

EP 1 505 041 and U.S. Pat. No. 7,316,740, which is hereby incorporatedby reference in its entirety, describe lithium silicate glass ceramicswhich can additionally contain 0 to 2 wt.-% ZrO₂ as well as 0.5 to 7.5wt.-% and in particular 0.5 to 3.5 wt.-% colouring and fluorescent metaloxides. EP 1 688 398 and U.S. Pat. No. 7,452,836, which is herebyincorporated by reference in its entirety, describe similar lithiumsilicate glass ceramics which are substantially free of ZnO and can alsocontain, in addition to the above-mentioned quantities of colouring andfluorescent metal oxides, 0 to 4 wt.-% ZrO₂, wherein however to achievehigh strengths smaller quantities of from 0 to 2 wt.-% ZrO₂ arepreferred. The glass ceramics are processed into the desired dentalrestorations in particular in the form of lithium metasilicate glassceramics by means of CAD/CAM methods, wherein a subsequent heattreatment effects the conversion of the metasilicate phase to thehigh-strength disilicate phase.

U.S. Pat. No. 6,455,451, which is hereby incorporated by reference inits entirety, relates to lithium disilicate glass ceramics which, inaddition to other components, can also contain transition metal oxides.It is proposed inter alia, in order to increase the refractive index ofthe glass matrix, to add small quantities of heavy elements such as Sr,Y, Nb, Cs, Ba, Ta, Ce, Eu or Tb. Thus, for example, CeO₂ and Tb₄O₇ canbe used in quantities of from 0 to 1 wt.-%, Nb₂O₃ and Ta₂O₅ inquantities of from 0 to 2 wt.-% and ZrO₂ and Y₂O₃ in quantities of from0 to 3 wt.-%. In one embodiment, Ta₂O₅ is said to be able to be presentin a quantity of from 0.5 to 8 wt.-%, even though the specific examplescontain at most 2.02 wt.-% of this oxide.

U.S. Pat. No. 5,176,961 and U.S. Pat. No. 5,219,799, which are herebyincorporated by reference in their entirety, disclose glass ceramics forexample for the production of crockery, which can contain as colorantsspecific transition metal oxides such as CeO₂, CO₃O₄, Cr₂O₃, CuO, Fe₂O₃,MnO₂, NiO and V₂O₅ in a quantity of from 0.01 to 7 wt.-%.

U.S. Pat. No. 5,507,981 and U.S. Pat. No. 5,702,514 which are herebyincorporated by reference in their entirety, describe processes forshaping dental restorations and glass ceramics that can be used in theseprocesses. These are in particular lithium disilicate glass ceramicswhich can contain 0 to 5 wt.-% colouring oxides such as SnO₂, MnO, CeO,Fe₂O₃, Ni₂O, V₂O₃, Cr₂O₃ or TiO₂.

Known glass ceramics based on lithium silicate often have opticalproperties which do not adequately satisfy the aesthetic requirements inparticular in connection with the use as dental materials. Thus knownglass ceramics often have an unfavourable refractive index. With glassceramics in particular there is the problem that the refractive indicesof the crystalline phase and of the glass phase usually differ markedlyfrom each other, which in most cases results in an undesired clouding ofthe glass ceramic. Similar problems exist for example in the case ofcomposites because the refractive indices of known glass ceramics andglasses usually differ from those of the polymer phase. There istherefore a need for glass ceramics based on lithium silicate therefractive index of which can be easily varied, but without the otherproperties being substantially impaired. Moreover, it is desirable thatsuch glass ceramics can be prepared and crystallized under conditionscomparable to those for customary glass ceramics.

DETAILED DESCRIPTION OF THE INVENTION

The lithium silicate glass ceramic according to the invention ischaracterized in that it comprises at least 8.5 wt.-% transition metaloxide selected from the group consisting of oxides of yttrium, oxides oftransition metals with an atomic number from 41 to 79 and mixtures ofthese oxides.

In general it is preferred that the transition metal oxide as componentof the glass ceramic according to the invention or of the glassaccording to the invention effects substantially no colour changecompared with a corresponding glass ceramic or a corresponding glasswithout the addition of this component. In particular, the transitionmetal oxide is colourless and/or non-fluorescent.

The transition metal oxide is preferably selected from the groupconsisting of oxides of Y, Nb, La, Ta, W and mixtures of these oxides.

Glass ceramics are preferred which comprise 8.5 to 30.0 wt.-%,preferably 9.0 to 25.0 wt.-%, in particular 9.5 to 20.0 wt.-%, preferred10.0 to 18.0 wt.-%, more preferred 10.5 to 16.0 wt.-% and most preferred11.0 to 15.0 wt.-% transition metal oxide selected from one or more ofthe above-named groups.

Surprisingly, by using the high content according to the invention oftransition metal with a high atomic number, the refractive index ofglass ceramics and glasses based on lithium silicate can be easilyadjusted without other properties being substantially impaired. Inparticular it was shown unexpectedly that the high content of transitionmetal with a high atomic number usually neither impedes the desiredcrystallization of lithium disilicate nor leads to the formation ofundesired secondary crystal phases, with the result that glass ceramicswith excellent optical and mechanical properties are obtained accordingto the invention.

A glass ceramic which comprises 54.0 to 80.0 and in particular 60.0 to70.0 wt.-% SiO₂ is further preferred.

In addition, a glass ceramic which comprises 11.0 to 19.0 and inparticular 12.0 to 15.0 wt.-% Li₂O is preferred.

It has proven particularly preferable if the glass ceramic comprises 0.5to 12.0 and in particular 2.5 to 6.0 wt.-% nucleating agents. Preferrednucleating agents are selected from P₂O₅, TiO₂, metals, e.g. Pt, Pd, Au,Ag, or mixtures thereof. Particularly preferably, the glass ceramiccomprises P₂O₅ as nucleating agent. Surprisingly, in particular P₂O₅ asnucleating agent effects the formation of desired lithium disilicatecrystals while largely preventing the formation of undesired secondarycrystal phases.

The glass ceramic according to the invention preferably comprises afurther alkali metal oxide in an amount of from 0.5 to 13.0, preferably1.0 to 7.0 and particularly preferably 2.0 to 5.0 wt.-%. The term“further alkali metal oxide” refers to alkali metal oxide with theexception of Li₂O. The further alkali metal oxide is in particular K₂O,Cs₂O and/or Rb₂O and is particularly preferably K₂O. It is assumed thatthe use of K₂O contributes to the strengthening of the glass networkcompared with the Na₂O used in conventional glass ceramics. It ispreferred that the glass ceramic comprises less than 2.0, in particularless than 1.0, preferably less than 0.5 wt.-% and particularlypreferably essentially no Na₂O.

It is further preferred that the glass ceramic comprises up to 6.0 wt.-%and in particular 0.1 to 5.0 wt.-% alkaline earth metal oxide, whereinthe alkaline earth metal oxide is in particular CaO, BaO, MgO, SrO or amixture thereof.

It is furthermore preferred that the glass ceramic comprises up to 6.0wt.-% and in particular 0.1 to 5.0 wt.-% ZnO.

The glass ceramic according to the invention can moreover also compriseadditional components which are selected in particular from oxides oftrivalent elements, further oxides of tetravalent elements, furtheroxides of pentavalent elements, melt accelerators, colorants andfluorescent agents.

A glass ceramic which comprises 0.2 to 8.0, in particular 1.0 to 7.0 andpreferably 2.5 to 3.5 wt.-% oxide of trivalent elements is preferred,wherein this oxide is selected in particular from Al₂O₃, Bi₂O₃ andmixtures thereof, and preferably is Al₂O₃.

The term “further oxides of tetravalent elements” refers to oxides oftetravalent elements with the exception of SiO₂. Examples of furtheroxides of tetravalent elements are ZrO₂, SnO₂ and GeO₂, and inparticular ZrO₂.

The term “further oxides of pentavalent elements” refers to oxides ofpentavalent elements with the exception of P₂O₅. An example of a furtheroxide of pentavalent elements is Bi₂O₅.

A glass ceramic which comprises at least one further oxide oftetravalent elements or one further oxide of pentavalent elements ispreferred.

Examples of melt accelerators are fluorides.

Examples of colorants and fluorescent agents are chromophoric orfluorescent oxides of d and f elements, such as the oxides of Sc, Ti,Mn, Fe, Ag, Ce, Pr, Tb, Er and Yb, in particular Ti, Mn, Fe, Ag, Ce, Pr,Tb and Er.

A glass ceramic which comprises at least one and preferably all of thefollowing components is particularly preferred:

Component wt.-% SiO₂ 54.0 to 80.0, in particular 60.0 to 70.0 Li₂O 11.0to 19.0, in particular 12.0 to 15.0 K₂O  0.5 to 13.5, in particular 1.0to 7.0 Al₂O₃  0.2 to 8.0, in particular 1.0 to 7.0 Alkaline earth oxide  0 to 6.0, in particular 0.1 to 5.0 ZnO   0 to 6.0, in particular 0.1to 5.0 Transition metal oxide  8.5 to 30.0, in particular 9.0 to 25.0P₂O₅  0.5 to 12.0, in particular 2.5 to 6.0 ZrO₂  0.1 to 4.0, inparticular 0.5 to 2.0 Colorants and fluorescent agents  0.1 to 8.0, inparticular 0.2 to 2.0.

The term “main crystal phase” used below refers to the crystal phasewhich has the highest proportion by volume compared with other crystalphases.

The glass ceramic according to the invention preferably has lithiummetasilicate as main crystal phase. In particular the glass ceramiccomprises more than 5 vol.-%, preferably more than 10 vol.-% andparticularly preferably more than 15 vol.-% of lithium metasilicatecrystals, relative to the total glass ceramic.

In a further preferred embodiment, the glass ceramic has lithiumdisilicate as main crystal phase. In particular the glass ceramiccomprises more than 5 vol.-%, preferably more than 10 vol.-% andparticularly preferably more than 15 vol.-% of lithium disilicatecrystals, relative to the total glass ceramic.

The lithium disilicate glass ceramic according to the invention ischaracterized by particularly good mechanical properties and can beproduced by heat treatment of the lithium metasilicate glass ceramicaccording to the invention.

It is also surprising that, despite its high content of a transitionmetal with a high atomic number, the lithium disilicate glass ceramicaccording to the invention usually has a good translucency and noamorphous-amorphous phase separation occurs in it and it can thus beused for example for the aesthetically pleasing coating of dentalrestorations.

The lithium disilicate glass ceramic according to the invention has goodmechanical properties and a high chemical resistance.

The invention also relates to a lithium silicate glass which comprisesthe components of the glass ceramic according to the invention describedabove. In respect of preferred embodiments of this glass, reference ismade to the preferred embodiments described above of the glass ceramicaccording to the invention. It was shown surprisingly that, despite thehigh content of transition metal with a high atomic number, homogeneous,clear glasses can be obtained which display no undesired phenomena suchas amorphous-amorphous phase separation or spontaneous crystallization.These glasses are therefore suitable for the preparation of the glassceramic according to the invention. Alternatively, a use for example asfiller for example in dental materials, in particular inorganic-organiccomposites, is also possible. A subject of the invention is also apolymerizable composition which comprises a glass ceramic or a glass asdescribed above and at least one polymerizable monomer. Suitablemonomers and further constituents of composites are known to a personskilled in the art.

A lithium silicate glass with nuclei which are suitable for theformation of lithium metasilicate and/or lithium disilicate crystals isparticularly preferred.

The glass according to the invention with nuclei can be produced by heattreatment of a correspondingly composed starting glass. By a furtherheat treatment the lithium metasilicate glass ceramic according to theinvention can then be formed, which in turn can be converted into thelithium disilicate glass ceramic according to the invention by furtherheat treatment. The starting glass, the glass with nuclei and thelithium metasilicate glass ceramic can consequently be seen asprecursors for the production of the high-strength lithium disilicateglass ceramic.

The glass ceramic according to the invention and the glass according tothe invention are present in particular in the form of powders orblanks, as they can easily be further processed in these forms. Theycan, however, also be present in the form of dental restorations, suchas inlays, onlays, crowns or abutments.

The invention also relates to a process for the preparation of the glassceramic according to the invention and the glass with nuclei accordingto the invention, in which a starting glass with the components of theglass ceramic or the glass is subjected to at least one heat treatmentin the range of from 450 to 950° C.

The starting glass therefore comprises at least 8.5 wt.-% oxide of atleast one transition metal as defined above. In addition, it preferablyalso comprises suitable quantities of SiO₂ and Li₂O, in order to makepossible the formation of a lithium silicate glass ceramic. Furthermore,the starting glass can also contain further components, such as aregiven above for the lithium silicate glass ceramic according to theinvention. Those embodiments are preferred which are also given aspreferred for the glass ceramic.

To prepare the starting glass, the procedure is in particular that amixture of suitable starting materials, such as carbonates, oxides,phosphates and fluorides, is melted at temperatures of in particularfrom 1300 to 1600° C., preferably 1450 to 1500° C., for 2 to 10 h. Toachieve a particularly high homogeneity, the obtained glass melt ispoured into water in order to form a glass granulate, and the obtainedgranulate is then melted again.

The melt can then be poured into moulds to produce blanks of thestarting glass, so-called solid glass blanks or monolithic blanks. Thecooling preferably takes place from a temperature of 500° C. with acooling rate of 3 to 5 K/min to room temperature. This is advantageousin particular for the production of stress-free glass products.

It is also possible to put the melt into water again in order to preparea granulate. This granulate can then be pressed, after grinding andoptionally addition of further components, such as colorants andfluorescent agents, to form a blank, a so-called powder green compact.

Finally, the starting glass can also be processed to form a powder aftergranulation.

The starting glass is then subjected, e.g. in the form of a solid glassblank, a powder green compact or in the form of a powder, to at leastone heat treatment in the range of from 450 to 950° C. It is preferredthat a first heat treatment is initially carried out at a temperature inthe range of from 500 to 600° C. to prepare a glass according to theinvention with nuclei which are suitable for forming lithiummetasilicate and/or lithium disilicate crystals. This glass can thenpreferably be subjected to at least one further temperature treatment ata higher temperature and in particular more than 570° C. to effectcrystallization of lithium metasilicate or lithium disilicate.

The at least one heat treatment carried out in the process according tothe invention can also take place within the framework of the pressingor sintering of the glass according to the invention or the glassceramic according to the invention onto a ceramic.

Dental restorations, such as inlays, onlays, crowns or abutments, can beprepared from the glass ceramic according to the invention and the glassaccording to the invention. The invention therefore also relates totheir use for the preparation of dental restorations.

In view of the above-described properties of the glass ceramic accordingto the invention and the glass according to the invention as itsprecursor, these are also suitable in particular for use in dentistry. Asubject of the invention is therefore also the use of the glass ceramicaccording to the invention or the glass according to the invention as adental material and in particular for the preparation of dentalrestorations or as a coating material for dental restorations, such ascrowns and bridges.

The invention is described in further detail below with reference toexamples.

EXAMPLES Examples 1 to 10 Composition and Crystal Phases

A total of 10 glasses and glass ceramics with the composition given inTable I (each in wt.-%) were prepared by melting corresponding startingglasses followed by heat treatment for controlled nucleation andcrystallization.

The starting glasses were firstly melted in a 100 to 200 g scale fromcustomary raw materials at 1400 to 1500° C. and transformed into glassfrits by pouring them into water. These glass frits were then melted asecond time at 1450 to 1550° C. for 1 to 3 h for the homogenization. Theobtained glass melts were poured into pre-heated moulds to produce glassmonoliths. These glass monoliths were transformed into glasses and glassceramics according to the invention by thermal treatment.

The crystal phases obtained after completion of all heat treatments weredetermined by high-temperature X-ray diffraction (HT-XRD) at thetemperatures listed in each case in Table I. Surprisingly, glassceramics with lithium disilicate as main crystal phase were alwaysobtained. Despite the high content of transition metals with a highatomic number, no secondary crystal phases were found with thesetransition metals.

Finally, the refractive indices of the respective glass phases weredetermined using Abbe refractometry (20° C., 589 nm). It was shown thatthe glass ceramics according to the invention have a much higherrefractive index than a comparison glass ceramic.

Although the present invention has been described in connection withpreferred embodiments thereof, it will be appreciated by those skilledin the art that additions, deletions, modifications, and substitutionsnot specifically described may be made without department from thespirit and scope of the invention as defined in the appended claims.

TABLE I 1 2 3 4 5 6 7 8 9 10 SiO₂ 67.4 58.4 66.4 63.5 67.0 61.8 66.466.4 61.8 54.5 K₂O 3.7 1.0 2.9 2.8 2.9 1.0 2.9 2.9 1.0 0.5 Li₂O 14.112.1 13.8 13.2 14.4 13.2 13.8 13.8 13.2 11.3 Al₂O₃ 3.2 1.0 2.9 2.5 1.02.9 2.9 1.0 0.5 P₂O₅ 3.1 2.5 4.0 4.0 4.0 5.0 4.0 4.0 5.0 3.2 WO₃ 8.5Nb₂O₅ 10.0 Ta₂O₅ 10.0 La₂O₃ 25.0 10.0 18.0 30.0 Y₂O₃ 14.0 10.0 18.0 CeO₂1.0 Er₂O₃ 0.3 Tb₄O₇ 0.4 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0100.0 100.0 Crystal Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ phase (s) Li₃PO₄ (800°) KAlSiO₄ (780°)Li₂SiO₃ (800°) Li₂SiO₃ LaPO₄ HT-XRD (800° C.) (820°) (800°) (700° C.)(800° C.) Refractive 1.5312 1.5547 1.5553 1.5494 1.5643 1.5403 1.54221.5586 index n_(d)

1. Lithium silicate glass ceramic, which comprises at least 8.5 wt.-%transition metal oxide selected from the group consisting of oxides ofyttrium, oxides of transition metals with an atomic number from 41 to 79and mixtures of these oxides.
 2. Glass ceramic according to claim 1,which comprises at least 8.5 wt.-% transition metal oxide selected fromthe group consisting of oxides of Y, Nb, La, Ta, W and mixtures of theseoxides.
 3. Glass ceramic according to claim 1, which comprises 8.5 to30.0 wt.-% of the transition metal oxide.
 4. Glass ceramic according toclaim 1, which comprises 9.0 to 25.0 wt.-% of the transition metaloxide.
 5. Glass ceramic according to claim 1, which comprises 9.5 to20.0 wt.-% of the transition metal oxide.
 6. Glass ceramic according toclaim 1, which comprises 10.0 to 18.0 wt.-% of the transition metaloxide.
 7. Glass ceramic according to claim 1, which comprises 10.5 to16.0 wt.-% of the transition metal oxide.
 8. Glass ceramic according toclaim 1, which comprises 11.0 to 15.0 wt.-% of the transition metaloxide.
 9. Glass ceramic according to claim 1, which comprises 54.0 to80.0 wt.-% SiO₂.
 10. Glass ceramic according to claim 1, which comprises60.0 to 70.0 wt.-% SiO₂.
 11. Glass ceramic according to claim 1, whichcomprises 11.0 to 19.0 wt.-% Li₂O.
 12. Glass ceramic according to claim1, which comprises 12.0 to 15.0 wt.-% Li₂O.
 13. Glass ceramic accordingto claim 1, which comprises 0.5 to 12.0 wt.-% nucleating agent, whereinthe nucleating agent is selected from P₂O₅, TiO₂, and/or metals. 14.Glass ceramic according to claim 1, which comprises 2.5 to 6.0 wt.-%nucleating agent, wherein the nucleating agent is selected from P₂O₅,TiO₂, and/or metals.
 15. Glass ceramic according to claim 1, whichcomprises further alkali metal oxide in an amount of from 0.5 to 13.5wt.-%, wherein the further alkali metal oxide is K₂O, Cs₂O and/or Rb₂O.16. Glass ceramic according to claim 1, which comprises further alkalimetal oxide in an amount of from 1.0 to 7.0 wt.-%, wherein the furtheralkali metal oxide is K₂O, Cs₂O and/or Rb₂O.
 17. Glass ceramic accordingto claim 1, which comprises further alkali metal oxide in an amount offrom 2.0 to 5.0 wt.-%, wherein the further alkali metal oxide is K₂O,Cs₂O and/or Rb₂O.
 18. Glass ceramic according to claim 1, whichcomprises up to 6.0 wt.-% alkaline earth metal oxide, wherein thealkaline earth metal oxide is CaO, BaO, MgO and/or SrO, and/or comprisesup to 6.0 wt.-% ZnO.
 19. Glass ceramic according to claim 1, whichcomprises 0.1 to 5.0 wt.-% alkaline earth metal oxide, wherein thealkaline earth metal oxide is CaO, BaO, MgO and/or SrO, and/or comprisesup 0.1 to 5.0 wt.-% ZnO.
 20. Glass ceramic according to claim 1, whichcomprises 0.2 to 8.0 wt.-% oxide of trivalent elements, wherein theoxide of trivalent elements is Al₂O₃ and/or Bi₂O₃.
 21. Glass ceramicaccording to claim 1, which comprises 1.0 to 7.0 wt.-% oxide oftrivalent elements, wherein the oxide of trivalent elements is Al₂O₃and/or Bi₂O₃.
 22. Glass ceramic according to claim 1, which comprises2.5 to 3.5 wt.-% oxide of trivalent elements, wherein the oxide oftrivalent elements is Al₂O₃ and/or Bi₂O₃.
 23. Glass ceramic according toclaim 1, which comprises at least one further oxide of tetravalentelements or at least one further oxide of pentavalent elements. 24.Glass ceramic according to claim 23, wherein the at least one furtheroxide of tetravalent elements comprises ZrO₂, SnO₂ or GeO₂.
 25. Glassceramic according to claim 23, wherein the at least one further oxide ofpentavalent elements comprises Bi₂O₅.
 26. Glass ceramic according toclaim 1, which comprises at least one of the following components:Component wt.-% SiO₂ 54.0 to 80.0 Li₂O 11.0 to 19.0 Al₂O₃  0.2 to 8.0K₂O  0.5 to 13.5 Alkaline earth oxide   0 to 6.0 ZnO   0 to 6.0Transition metal oxide  8.5 to 30.0 P₂O₅  0.5 to 12.0 ZrO₂  0.1 to 4.0Colorant and fluorescent agent  0.1 to 8.0.


27. Glass ceramic according to claim 1, which comprises at least one ofthe following components: Component wt.-% SiO₂ 60.0 to 70.0 Li₂O 13.0 to17.0 Al₂O₃  1.0 to 7.0 K₂O  1.0 to 7.0 Alkaline earth oxide  0.1 to 5.0ZnO  0.1 to 5.0 Transition metal oxide  9.0 to 25.0 P₂O₅  2.5 to 6.0ZrO₂  0.5 to 2.0 Colorant and fluorescent agent  0.2 to 2.0.


28. Lithium silicate glass, which comprises the components of the glassceramic according to claim
 1. 29. Lithium silicate glass according toclaim 28, wherein the glass comprises nuclei which are suitable forforming lithium metasilicate and/or lithium disilicate crystals. 30.Glass ceramic according to claim 1, which is present in the form of apowder, a blank or a dental restoration.
 31. Glass according to claim29, which is present in the form of a powder, a blank or a dentalrestoration.
 32. Process for the preparation of the glass ceramicaccording to claim 1, in which a starting glass with the components ofthe glass ceramic is subjected to at least one heat treatment in therange of from 450 to 950° C.
 33. Process for the preparation of theglass according to claim 29, in which a starting glass with thecomponents of the glass is subjected to at least one heat treatment inthe range of from 450 to 950° C.
 34. Process of using the glass ceramicaccording to claim 1, as dental material.
 35. Process of using the glassceramic according to claim 34, wherein the dental material is used forcoating dental restorations, for the preparation of dental restorationsor as filler in inorganic-organic composites.
 36. Process of using theglass according to claim 28, as dental material.
 37. Process of usingthe glass according to claim 36, wherein the dental material is used forcoating dental restorations, for the preparation of dental restorationsor as filler in inorganic-organic composites.
 38. Process of using theglass according to claim 29, as dental material.
 39. Process of usingthe glass according to claim 38, wherein the dental material is used forcoating dental restorations, for the preparation of dental restorationsor as filler in inorganic-organic composites.
 40. Glass ceramicaccording to claim 1, wherein the refractive index is at least 1.53. 41.Glass ceramic according to claim 1, wherein the refractive index is atleast 1.55.
 42. Glass according to claim 28, wherein the refractiveindex is at least 1.53.
 43. Glass according to claim 28, wherein therefractive index is at least 1.55.